This invention relates to polyester compositions, and more particularly to compositions containing poly[ethylene naphthalate] (PEN), polyt[ethylene terephthalate] (PET) and poly[butylene terephthalate] (PBT) which display excellent crystallisation behaviour, useful for improved processing and moulding characteristics.
It is known that medium content PET/PEN compositions (compositions with PET:PEN ratios around 50:50) are usually amorphous in nature. Tile range of compositions which display this amorphous behaviour is generally accepted to be around PET:PEN=20:80 to PET:PEN=80:20, as described by two PEN manufacturers - Shell (see FIG. 1 of presentation to xe2x80x9cBevPakxe2x80x9d conference, Spring 1995, USA) and Hoechst-Trevira (page 4 of Polyclear(copyright) N technical literature). The same observations on the amorphous/crystalline nature of PET/PEN compositions were also made by Lu and Windle (see FIG. 2 in Polymer 36 (1995), pages 451-459) and Andresen and Zachmann (Colloid and Polymer Science 272 (1994), page 1352).
The disadvantages of this behaviour are that the use temperature of compositions in the 80/20-20/80 region is substantially reduced and tile mechanical properties may be compromised. These drawbacks arise from the difference in behaviour of the amorphous phase and the semi-crystalline phase in different PET/PEN compositions. In the case of bottle manufacture using PET/PEN copolymers and blends, U.S. Pat. No. 5,628,957 (to Continental PET Technologies Inc.) states that mid-range PET/PEN compositions containing 20-80% PEN are substantially amorphous and describes the use of an additional strain-hardeniable (ie. ciystallisable) layer for these mid-range PET/PEN bottles.
It would be advantageous therefore to be able to nucleate these medium content PET/PEN compositions to take advantage of the higher thermal capability and mechanical properties offered by a semi-crystalline PET/PEN composition in the otherwise amorphous 80/20-20/80 region.
In contrast to PET/PEN behaviour, PEN/PBT compositions possess crystallinity across the entire composition range, as shown by the work of Guo and Zachmann (Polymer Preprints (ACS, Division of Polymer Chemistry) 37 (1996), pages 829-830). Crystalline melt temperatures (Tm) are observed for both the PEN and PBT components, varying with composition. However, no suggestion is made in this article relating to the inclusion of PET in PEN/PBT blends as in the present invention.
PET/PBT compositions also show crystallinity across a wide composition range, including 50:50 compositions, as documented in GB 1,398,598 (to General Electric) for example. This document presents Tm values for either PET or PBT or a PET/PBT alloy. However, once again no mention is made of the inclusion of PEN polymer to produce a PET/PEN/PBT composition of the present invention.
Hence, the use of PBT to promote crystallinity development in typically amorphous medium content PET/PEN compositions has not been reported before. We have found that PBT can be used for this purpose. Furthermore, the effectiveness of PBT is surprisingly highxe2x80x94even medium content PET/PEN compositions in the 50/50 region undergo crystallisation so readily following the addition of PBT that a wide range of PET/PEN/PBT compositions possess a higher crystallisation temperature than unmodified PET.
The present invention provides a composition comprising from 20 to 70 parts by weight of polyethylene tereplitlialate] (PET), from 20 to 70 parts by weight of poly[ethylene naphthalate] (PEN) and from 10 to 33 parts by weight of poly[butylene terephthalate] (PBT), the total being 100 parts by weight, in the form of a polymer blend and/or copolymer.
The invention also provides use of PBT to promote crystallisation in a composition comprising PET and PEN, in the form of a polymer blend and/or copolymer.
The composition comprising PET, PEN and PBT undergoes fast crystallisation on cooling from the melt.
The proportion of each polyester in the composition is in the range 20-70 parts for PEN, 20-70 parts for PET and 10-33 parts for PBT where the total proportion in the composition as a whole is 100 parts. The content of PBT is preferably 10 to 25, more preferably 15 to 20 parts by weight.
In some applications, it is common practice to modify PET and PBT structures by introducing a small proportion of alternative monomer in place of the ethylene glycol, butylene glycol or terephthalic acid monomers which are the constituent monomers of PET and PBT. Examples of such alternative monomers include isophtlialic acid, cycloliexane dimethanol and the like. In the present invention, PET is specified to consist of  greater than 90 mole % ethylene units and  greater than 90 mole % terephlithalate units. Similarly, PBT is specified to consist of  greater than 90 mole % butylene units and  greater than 90 mole % terephthalate units.
The PEN is preferably polyl[ethylene naphthalene-2,6-dicarboxylate].
The PET:PEN mole ratio is generally from 20:80 to 80:20, preferably from 40:60 to 60:40.
Compositions of the invention display crystallisation on cooling from the melt at all PET/PEN/PBT ratios. This is advantageous in both processing, where faster cycle times may be used, and in the properties of the final product on which the overall level and type of crystallinity impinges. These propelties may include heat distortion temperature, hot-fill temperature, mechanical properties, barrier properties, dimensional stability, mould release, surface appearance and the like. The fact that PBT is a polymer rather than a heterogeneous nucleating additive means that obtaining good mixing with PET and PEN polymers presents no problems, especially when compared with the care that has to taken to obtain good mixing with heterogeneous nucleating additives.
The compositions may consist essentially of the PET, PEN and PBT.
The use of common additives, typically at levels up to 1 wt %, does not unduly effect the behaviour of the compositions of the invention. Such additives include process additives such as flow modifiers and the like and product-dependent additives such as pigments and the like. The invention is also effective for filled compositions containing fibrous or particulate fillers including materials such as glass fibres, carbon fibres, mica, carbon black, talc and the like.
The composition can be formed into shaped products, e.g. moulded articles such as bottles, and also fibres and films. Compositions for bottles may contain dyes at less than 1 wt % but generally no other additives. Compositions for fibres or films may contain dyes at less than 1 wt %, processing aids at less than 2 wt % and fillers at less than 10 wt %, generally less than 5 wt %. (all percentages based on total weight).
The invention is illustrated by the following Examples.